Engineering Notes Index

Minnesota/Wisconsin Engineering Notes
Summer 1997

Proper Fans Needed for Best Grain Drying Cost and Performance

Bill Wilcke, Minnesota Extension Engineer

The fan is probably the most critical piece of equipment on grain storage bins and bin-type grain dryers. The fan determines airflow per bushel of grain, which in turn determines how well drying and aeration systems perform, and how much it costs to operate these systems. And since fans are fairly expensive to purchase and to operate, and they last a long time (ten to twenty years), it is worth putting some time and effort into selecting the proper fan.

The key parts to a fan are the impeller - the part that moves the air, and the motor that drives the impeller. Most fans are identified by the size of the impeller (diameter in inches) and by the horsepower rating of the motor (hp).

When air moves through grain, it passes through the tiny air spaces between seeds, and resistance to airflow is affected by the size of these airspaces, the velocity of the air, and the length of the airflow path. Thus, total airflow resistance through grain is determined by grain depth, seed size, amount of broken grain and chaff that are present, and the total amount of air that you're trying to move through the bin. Airflow resistance is reflected in the amount of static pressure that the fan works against to push or pull air through grain. In the United States, static pressure is measured in inches of water (in. water) and airflow is measured in cubic feet of air per minute (cfm).

The performance of most fans has been tested using standard laboratory procedures. These tests yield a fan performance chart or graph that shows how much air the fan can move against various static pressures. This information is essential for selection of the proper fan for a given grain bin and it should be available from the fan dealer or manufacturer. Don't buy a fan for which performance information is not available.

Fan selection can be confusing, because there are several different types and lots of different brands from which to choose. Some types of fans work better in certain situations than others, so you can narrow your search a bit if you understand the differences between basic fan types.

• Axial-flow fans. The impeller is usually mounted directly on the motor shaft and both the impeller and motor are in the airstream inside a barrel-shaped housing. Air moves in one end of the housing and out the other in line with the motor shaft. These fans are the least expensive type, but they are also very noisy. They are the most efficient type of fan in low static pressure situations (less than about 4 in. water) - for example, where there is a shallow grain depth, low airflow per bushel, or large-seeded crop like corn or soybeans. Performance of axial-flow fans drops off rapidly at pressures above 5 in. , and eventually stall out (deliver no air) at very high pressures.

   

• Centrifugal fans. These are available in low-speed (1700 to 1800 rpm) or high-speed (3400 to 3600 rpm) models. The high-speed fans generally have a smaller impeller and deliver less airflow than the low-speed fans, but they work against much higher pressures. Centrifugal fans are sometimes called "squirrel-cage" fans or "blowers", and one company calls its high-speed centrifugal fan an "air pump." Air enters the center of the impeller, or wheel, through one or both ends, and then moves out to the edge of the wheel and leaves the wheel at a tangent to the blades. These fans are more expensive than axial-flow fans, but they are much quieter and they are needed in high-pressure situations. High static pressure can be expected for small-seeded crops like wheat or canola, large grain depths, or high airflow per bushel. Centrifugal fans will work at low pressures, but they are not as efficient as axial-flow fans in low-pressure situations.

   

• In-line centrifugal fans. These fans are a hybrid between centrifugal and axial-flow fans; they have the wheel-type impeller of centrifugal fans and the motor mounted parallel to the motor shaft inside an axial-flow type housing. The price, noise-level, and performance of these fans is somewhere between that of centrifugal and axial-flow fans. They work well in moderate pressure situations.

   

The steps in selecting the proper fan for a given grain bin are:

1. Determine bin diameter, normal grain depth, and the crop that will be dried or stored most often. Recognize that if a bin will be used for both a small-seeded and a large-seeded crop, (wheat and corn, for example) that the airflow and static pressure will be much different for the two crops.

    

2. Decide what airflow per bushel you would like to deliver; 0.1 to 0.2 cfm/bu is common for dry grain aeration, and 1 cfm/bu is common for natural-air grain drying.

    

3. Get the fan performance information (cfm vs. in. water) for several different types and brands of fans.

    

4. Select the fan or fans that will give you the desired airflow per bushel.

    

5. Buy the fan that will give you the airflow you need at the lowest ownership and operating cost.

    

For more information on selecting fans, order a copy of Selecting Fans and Determining Airflow for Crop Drying, Cooling, and Storage, FO-5716 from the University of Minnesota Extension Service Distribution Center, telephone 612-625-8173. Or get a copy of our user-friendly fan selection computer program, FANS, by downloading it from our web site (http://www.bae.umn.edu/extens/harvest.html) or by contacting Bill Wilcke (wilck001@umn.edu).

The University of Minnesota is an equal opportunity educator and employer.

This page is part of the Biosystems and Agricultural Engineering Department web at http://www.bae.umn.edu/